Test UTF-8 handling of scripts and data

[historic_bruno]

Now that we support UTF-8 script and data files (e.g. JSON), it would be nice to test that we don't break it by accident. We already ​test pre-loaded JSON and script strings, but not loading them from files (beware the confusing LoadScript() call takes a filename but only for debugging). It shouldn't be any more complicated than figuring out how to add the VFS init stuff to the tests and then adding a few test files. I think we should test UTF-8 with and without ​BOM, since that seems likely to cause breakage.